JPS61201637A - Production of base material for optical fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a base material for an optical 7 eyer, and particularly relates to a method for manufacturing a base material O11 for a high-quality silica-based optical fiber containing fluorine in the cladding portion. .

[Conventional technology]

The second layer shows the refractive index distribution structure of a typical single mode optical fiber. Conventionally, in order to form such a refractive index distribution, there have been many methods of adding substances that increase the refractive index to the core portion. G・0□ as an additive to increase the refractive index
, P2O5,*l, Os, etc. are often used; however, when these oxides are used, ■ the transmission loss at the tip increases due to an increase in Rayleigh scattering; Problems such as bubble generation and crystal phase precipitation caused by oxides are likely to occur, and (2) the glass base material, which has a large coefficient of thermal expansion, is likely to break. Therefore, it is desirable that the amount of dopant added to the glass base material be small.

For this reason, a dopant that lowers the refractive index is added to the cladding part, such as B20. In some cases, methods of increasing the refractive index difference between the core and the cladding by adding fluorine or the like may be used.

However, B2O5 increases the coefficient of thermal expansion of silica glass and also has absorption loss specific to long wavelength regions. Therefore, it is desirable to use fluorine as the refractive index lowering component. On the other hand, as a manufacturing method for optical fibers, the WAD method (vapor phase attachment method), which forms a porous glass body by flame hydrolysis reaction, or The OVPO method (external CVN method) is known as an economical method with excellent productivity.However,
It is extremely difficult to add a sufficient amount of fluorine gold into quartz glass using a method using flame hydrolysis such as the MAD method or the OVPO method. For example, JP-A-55-15
Publication No. 682 describes a method of adding fluorine to a glass base material, but according to this method, the decrease in the refractive index due to the addition of fluorine is only about 0.2 to 0.3% at most. There is a limit to the amount of fluorine that can be added.

Furthermore, in JP-A No. 55-67533, fluorine is efficiently added to a laminate of glass particles formed by flame hydrolysis by heating it in an atmosphere of fluorine compound gas. A method has been proposed. However, in the method described in the above publication, fluorine is added almost uniformly to the glass particle laminate, so only fluorine is used to form a refractive index distribution that has a sufficient function as an optical waveguide as shown in Figure 2. What to do is...

Therefore, we applied a method of total synthesis of a porous glass body using a flame hydrolysis reaction, which forms a refractive index distribution that has a sufficient function as an optical waveguide and has excellent productivity. As a manufacturing method, a method using the apparatus schematically shown in the third factor is considered.

A glass rod 1 corresponding to a core portion, which is attached to a rotating and pulling device 2, is rotated while being gradually pulled upward, and at the same time, a burner 3 for synthesizing glass particles is generated on the side surface of the glass rod 1. Glass particles are deposited to form a porous glass layer 4 corresponding to a cladding part. The glass fine particles are placed in a burner 3 for glass fine particle synthesis in B2.
02, 51014, etc. are simultaneously supplied and formed by flame hydrolysis reaction. 5 is a reaction vessel, and 6 is an exhaust port. By heating the thus formed composite of the glass rod and the porous glass layer in a gas atmosphere containing fluorine, fluorine is incorporated into the porous glass layer and the porous glass layer becomes transparent. It can be vitrified and used as an optical fiber preform having a refractive index distribution as shown in FIG.

[Problems that the project is trying to solve]

However, the third factor device mentioned above? In this method, when the glass rod corresponding to the core portion is drawn to a predetermined diameter in advance, it is often heated in an atmosphere containing water vapor, and the surface of the glass rod is likely to be contaminated with OH groups. In particular, when the glass rod is stretched using a flame formed by combustion gas containing hydrogen atoms, contamination by OH groups on the surface of the glass rod is significant. Furthermore, when forming a porous glass layer corresponding to the cladding part, the surface of the glass rod may be contaminated with OH groups due to water vapor generated from the flame of the burner for synthesizing glass particles.

In this way, when a base material whose surface of a glass rod corresponding to the core is contaminated with OH groups is spun into an optical fiber, the light propagating through the optical fiber is affected by absorption loss due to the presence of OH groups, and transmission is reduced. Loss characteristics deteriorate. In particular, when the optical fiber is used as a single mode fiber, the power distribution of light propagating in the single mode optical fiber extends widely to the cladding, so OH group contamination near the boundary between the core and the cladding The influence of the layer is particularly significant, and the deterioration of transmission loss characteristics is significant.

By the way, as a result of intensive research, the present inventors have found that in a method of incorporating fluorine into a porous glass body by heating the porous glass body in an atmosphere containing fluorine, the amount of fluorine added is determined by the amount of fluorine or fluorine compounds in the atmosphere. concentration,
It has been found that the heating temperature and the turn density of the porous glass body are highly dependent.

For example, if the refractive index reduction rate of fluorine-doped silica glass expressed as a percentage is Δn (FJ), then y4 25X10' I an (F) lo oc P X exp (
--) T. Here, P is the partial pressure of the fluorine compound gas used as the atmospheric gas, and R is the Boltzmann constant (1,987 Caj
/lleg-nor), D represents the absolute temperature of the atmosphere. Further, FIG. 4 shows the relationship between the bulk density and the refractive index difference. In the case of Figure 4, 1200' CSF6th partial pressure 0.02
Porous glass bodies with different densities were kept in a He (the others were He) furnace for 3 hours and then turned into transparent vitrification at 1650°C, and the change in refractive index relative to quartz was determined.

As can be seen from FIG. 4, even if the porous glass base material is exposed to an atmosphere of the same temperature, time, and fluorine compound gas partial pressure, there is a large difference in the amount of fluorine added depending on the bulk density of the porous glass base material.

Therefore, by intentionally creating a bulk density distribution in the radial direction in the porous glass base material, it is possible to give the added fluorine a concentration distribution, that is, a refractive index distribution, in the radial direction.

10,000, the dehydration property of the porous glass base material, that is, the amount of residual OH when the porous base material is exposed to a certain temperature atmosphere containing a certain dehydrating agent and then turned into transparent vitrification at a high temperature is determined by the amount of residual OH of the porous glass base material. It is well known that force largely depends on density, and that the higher the bulk density, the greater the amount of residual OH. In particular, when forming the concentration distribution of the amount of fluorine contained in the porous glass base material by giving a bulk density distribution as described above, it is necessary to give sufficient consideration to the dewatering method VC of the portion with high bulk density.

In consideration of the above points, an object of the present invention is to provide a method for manufacturing an optical fiber with extremely low residual OH content, the core of which is substantially made of quartz glass, and the cladding portion of which is made of fluorine-containing quartz glass, and which has extremely low transmission loss. It's there.

[Means to solve all problems]

The present invention provides a porous glass layer with a high bulk density surrounding a starting material, and a porous glass layer with a low bulk density surrounding the high bulk density porous glass layer. After forming by depositing, the above starting material is removed to form a hollow porous glass body, and the porous glass body is subjected to dehydration,
By adding fluorine and applying heat treatment for transparent vitrification, a hollow transparent glass base material with a fluorine content distribution in the radial direction is formed, and then a glass etching agent is applied inside the hollow transparent glass base material. This is a method for producing an optical fiber preform in which the inner wall of the 51 hollow transparent glass preform is completely smoothed by heating it while flowing a containing gas, and the preform is heated to make it solid.

A particularly preferred embodiment of the present invention includes the above method in which the glass etching agent is fluorine or a compound gas containing fluorine.

The details will be explained below. The methods and examples described below are merely illustrative of the present invention, and the position, relative arrangement, size, shape, etc. of each part in the drawings do not limit the present invention in any way.

FIG. 1 shows one embodiment of the method for forming a porous base material according to the present invention. In the figure, 7 is a cylindrical or cylindrical refractory starting material, which is attached to an 80-turn pulling device. ing. Reference numeral 9 indicates a burner for forming the high density porous glass layer 11, and reference numeral 10 indicates a low bulk density porous glass layer 12'.
1 is a burner for forming the porous glass body, 13 is a quartz pipe for holding a porous glass body, 14 is a reaction vessel, and 15 is an exhaust port. 16 is a pin for easily attaching the holding quartz pipe to the starting material 1. The burners 9 and 10 are supplied with combustible gases such as H2 and hydrocarbons, O, quartz glass raw materials such as 5iGj4, and the like. Since the burner 9 forms a porous glass layer with a high bulk density, its arrangement and the flow rate of the gas raw material to be supplied are adjusted so that the surface temperature on which the glass fine particles are deposited becomes high. In the flame of each burner, glass particles are formed due to the flame hydrolysis reaction of the glass raw material, and these glass particles are attached to the holding quartz pipe 130.
The starting material 7 is deposited starting from the outer circumference of the starting material 7 in the axial direction by gradually pulling it upward while rotating the starting material 7. By depositing the glass fine particles to a predetermined length on the outer periphery of the starting material 7 in this way, the starting material 7
A porous glass base material surrounding this is formed. Thereafter, by pulling out the starting material 7 from the porous glass preform, a cylindrical (hollow) porous glass preform held by the holding quartz pipe 15 can be obtained. The hollow porous glass base material consists of a high bulk density part 11 formed by the burner 9 in the inner layer part and a low bulk density part 12 formed by the burner 10 surrounding this part.

The starting material 7 used in the method of the present invention may be made of carbon, silicon carbide, quartz glass, alumina, zirconia, etc., which may cause corrosion or deformation due to the flame of the burner 9 or 10 during glass particle synthesis. It is necessary that the material is difficult to use. In particular, zirconia is a desirable material because it is easy to pull the starting material 7 out of the porous glass body.

In addition, although an example is shown in which only two burners are used as glass particle synthesis burners in the first garden, stability can be improved by increasing the number of burners as necessary or by installing a heating burner for bulk density adjustment. It may also be possible to form a porous matrix.

Next, the cylindrical porous glass base material prepared as described above is subjected to dehydration and fluorine addition transparent treatment. The reason why the porous glass base material was made into a cylindrical shape is that, as mentioned above, in a solid porous glass base material, dehydration in the high bulk density area diffuses from the outside of the base material through the entire porous glass layer. (This is because it is difficult or takes a long time to carry out the dehydration process, as it can only be carried out using a dehydrating agent. In other words, by making it cylindrical, the high bulk density part of the inner layer can come into direct contact with the dehydrating agent flowing through the hollow part. is carried out efficiently.

FIG. 5 and FIG. 46 are diagrams showing embodiments of the method of dehydration and fluorine addition to produce transparent vitrification. 17 is a cylindrically arranged furnace 1B for heating a porous glass base material; 19.20 is a gas inlet; 21 is an exhaust port; 22.22
' is a holding pipe and a holding partner for holding the porous glass base material through the holding quartz pipe 13i, and is easily attached using a holding quartz pipe pin, etc.
It's becoming a sea urchin. The holding partner 22 is provided with a gas passage 23.

First, keep the inside of the furnace at around 1100℃ and gas inlet 19.2.
A dehydrating agent and a diluent gas such as Cl2 and He are introduced from 0 to sufficiently increase the OH groups remaining in the porous glass base material. At this time, in order to sufficiently expose the inner layer part 11 of the high bulk density part to C/2, in the method of the fifth factor, the gas flow rate flowing in from the gas inlet 20 is adjusted to the lower gas inlet 11.
The flow rate of the gas introduced from 9.

In the method shown in Fig. 6, in order to facilitate the flow of gas into the hollow part of the porous glass base material from the lower gas inlet 19, a part of the holding partner 22' is made to flow through the hollow part of the base material. Dehydration processing can be carried out more efficiently by providing a gas passage 23 or the like.

Thereafter, while increasing the furnace temperature, a fluorine compound gas such as SF6. OF4゜02F6.
A diluent gas such as SiF4 or H6 is introduced to perform fluorine addition and transparency treatment. Completely transparent vitrification can be achieved by raising the furnace temperature to about 1650°C. Note that it is also possible to separate the fluorine addition treatment and the transparent vitrification treatment, such as performing the fluorine 815 treatment at a temperature lower than the transparent vitrification temperature, and then increasing the furnace temperature to about 1650° C. to effect transparent vitrification.

FIG. 7 shows a schematic radial refractive index distribution of the base material thus obtained. At the stage of porous glass, the inner layer of the base material has a high bulk density, an extremely small amount of fluorine added, and a refractive index almost equal to that of pure silica glass.

In addition, in the above description of the dehydration fluoridation transparent vitrification process, an example of a furnace (soaking furnace) with a soaking temperature distribution in which the furnace length is approximately equal to or longer than the length of the porous glass body was shown, Even a method in which the porous glass base material is passed through a ring-shaped furnace (zone furnace) with a short furnace length does not deviate from the spirit of the present invention.

Next, a method for solidifying the hollow transparent glass base material obtained above will be described. Normally, when a tubular quartz pipe is solidified, it is heated from the outer periphery using a flame or an electric furnace, and in some cases, the pressure is reduced above the hollow part. However, in the method of the present invention, when producing a hollow porous glass body, it is necessary to pull out the entire starting material. is inferior, and tends to cause bubbles to form during solidification. Furthermore, there is a possibility that impurities such as moisture may adhere to the surface of the inner layer of the base material from the time of transparency to the time of solidification, which may cause deterioration of transmission loss.

Therefore, in the present invention, prior to solidification, a glass etching agent such as F-containing compound gas such as 02 is poured into the hollow part of the base material as necessary, and the base material is completely heated from the outside. It is effective to smooth the surface of the inner layer of the base material and remove moisture. Also, 02 is 02 on the inner surface of the glass.
By reducing the number of defects, it is effective to reduce deterioration in transmission characteristics caused by 02 defects.

As a glass etching agent, fluorine (F2) or CtF4.8F6. QC/2F2.02F6. BF
,, NF, a compound gas containing fluorine (F), etc. can be used.

The temperature at this time is generally about 9GO51300°C. Moreover, solidification can be performed at about 1800 to 2000°C. However, these temperatures vary depending on the size of the base material, etc., and do not limit the present invention in any way.

〔Example〕

Example 1 A porous glass base material was prepared using a zirconia tube having a diameter of 15 square meters as a starting material and having the structure shown in FIG. 5LO for burner 9 for forming the inner layer (high bulk density) porous glass layer
1a 50 CC 7 minutes, 628137 minutes, 0□ 10bu 7 minutes was supplied, and the outer layer (low catastrophe density) SiC/4B 00007 minutes, H22 was supplied to the burner 10 for forming the porous glass layer.
0J 7 minutes, 0250 supplied 7 minutes. Starting material f5
A porous glass preform having an outer diameter of 100111φ and an inner layer thickness of about 5 mm was formed by gradually pulling up at a speed of 60 mm/hour while rotating at 0 rpm. At this time, the average bulk density of the low bulk density part of the outer layer is 0.25j1101', and the average force of the high bulk density part of the inner layer is 1.01/
It was rlyr'. After the starting material was extracted from the porous base material, it was subjected to dehydration and fluorine addition transparency treatment using the configuration shown in FIG. Dehydration was carried out at a temperature of 1100°C, using C12100C1; 7 minutes from the lower gas inlet 19;
7.51 iJ/minute was flowed, and O/22J/minute was flowed from the upper gas inlet 120 and held for 3 hours for dehydration.

After that, the temperature was increased by 5℃ from 110Q℃ to 1600℃.
SF 4 20 G DC 7 minutes, Ha 8 A/min from the lower gas inlet 19.
By flowing 0.21/min of aH from the upper gas inlet 20, transparent vitrification was performed while the fluorine was contained in the porous base material of the fluorine tube.

The refractive index distribution of the obtained cylindrical transparent glass base material is shown in FIG. 8. In the figure, a is [QIII, l) is 14■, and 0 is 4ass.
Met. Also, the percentage shown in the figure indicates the difference from the refractive index of pure silica glass.

Quartz dummy pipes on both ends of the transparent glass base material.

After welding, it was inserted into an electric furnace at 1500°C, and sy6 was heated at 300α/min o2. from one end of the quartz pipe. 5G
G 007 minutes, held for 20 minutes and the inner layer part of the base material yo,
It was etched by a thickness of sam and smoothed, and then the furnace temperature was raised to 11,900°C to solidify it. the result,
Outer diameter: 45 ■φ Core diameter: 3 ■φ Difference in relative refractive index between core and cladding: 0.29% (some fluorine is added to the core, and the relative refractive index of the core relative to quartz glass is 0.03%). , the relative refractive index difference of the cladding part with respect to quartz glass is -0.32
%) was obtained for a single mode optical fiber preform.

The preform was spun to an outer diameter of 125 μmφ, and the transmission loss was measured to be 0.45 at a wavelength of 1.3 μm.
dB/Ax, 0.28 at 1.55am (18/1m
01 at 13857111 (absorption loss increase is 46B/la
1 and had excellent transmission loss.

Comparative Example 1 Umbrella density in the center ~1. Q&/Wound 3 has a high density area with an outer diameter of 5■φ and a density area surrounding the cough high density area *p ~ 0
．． 251/am3 A solid porous glass body having a low bulk density part with an outer diameter of 1408φ was formed, and the atmosphere gas was C/22 J/min, H・10bu7.
After being kept in an electric furnace at 1100°C for 6 hours and heated and dehydrated, the temperature was further increased from 110G°C to 1°C.
While increasing the temperature to 600℃ at a rate of 5℃/min,
SF6200 (X: /min He8t
3/min, and transparent vitrification was performed while allowing fluorine to be incorporated into the porous base material. As a result, we were able to obtain a base material in which almost no fluorine was contained in the high bulk density area, and only 0.3% of fluorine was contained in the low bulk density area in terms of the degree of decrease in refractive index. When the water content was measured using an infrared spectrometer, ~200 ppm of water remained in the high bulk density area, and dehydration could not be performed at all.

Comparative Example 2 When a hollow transparent glass body was formed in the same manner as in Example 1 and then solidified without smoothing, many air bubbles remained in the center of the base material, making spinning impossible.

Comparative Example 3 A hollow transparent glass body was formed in the same manner as in Example 1, and then the 02 flow was not completely flowed during the smoothing treatment, and the transmission loss after spinning was measured to be about 0.0 at all wavelengths. 'Sd
The loss was higher than that of Example B/A2I.

〔Effect of the invention〕

As is clear from the above explanation and the results of the examples, the method of the present invention improves the distribution of fluorine contained in the porous glass base material by adjusting the bulk density distribution of the inner layer and outer layer of the porous glass base material. In addition, it is possible to prevent OH group adhesion as much as possible during dehydration, fluorine addition, transparency process, and solidification process, so the core is quartz and the cladding is made of quartz glass containing fluorine, so the content of OH groups is low. A high-quality silica-based optical fiber base material can be obtained.

Brief explanation of the four factors Figure 1 A schematic diagram showing an embodiment of the method for forming a porous glass body according to the present invention, Figure 2 A graph showing an example of the refractive index distribution of a single mode optical fiber, Figure 3 Figure 4 shows an example of the conventional method for manufacturing a cladding layer of an optical fiber base material containing fluorine in the cladding part. Figure W45 shows the relationship between the bulk density of a porous glass body and the amount of fluorine added. and Fig. 6 is a diagram showing an embodiment of the method of dehydrating, fluoridating, and transparentizing a porous glass preform according to the present invention, and Fig. 7 is a refractive index of a hollow transparent glass preform obtained by the present invention. Distribution graph, FIG. 8 A graph of the refractive index distribution of the hollow transparent glass base material obtained in the example.

Claims (1)

[Claims] 1. A porous glass layer with a high bulk density surrounding the starting material;
After forming a low bulk density porous glass layer surrounding the high bulk density porous glass layer by depositing glass fine particles on the starting material, removing the starting material to form a hollow porous glass body, By subjecting the porous glass body to dehydration, fluorine addition, and heat treatment for transparent vitrification, a hollow transparent glass base material having a fluorine content distribution in the radial direction is formed, and then the hollow transparent glass base material An optical fiber preform characterized in that the inner wall of the hollow transparent glass preform is smoothed by heat treatment while flowing a gas containing a glass etching agent inside the preform, and the preform is made solid by heating. Production method. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the glass etching agent is fluorine or a compound gas containing fluorine. 3. The optical fiber preform according to claim 1 or 2, wherein the gas flowing inside the hollow transparent glass preform contains O_2 when smoothing the inner wall of the hollow transparent glass preform. Production method.